AutoDesk AutoCAD API to write 3rd party tools for AutoCAD. ObjectARX is also used in RealDWG package. RealDWG library is a subset of ObjectARX to handle AutoCAD database files (dwg and dxf files).
ObjectARX is an API published by Autodesk, Inc. The original ObjectARX SDK is a native API. Autodesk also publishes a managed equivalent called the ObjectARX .NET SDK. ObjectARX is used primarily for creating add-ons that work in-process with AutoCAD and AutoCAD-based applications. A subset of the API is used by standalone applications using the RealDWG SDK for accessing DWG and DXF format files without AutoCAD.